The present invention relates to an optical pick-up for use in an information recording/read apparatus.
There are various information recording/read apparatuses by optical means, including a video disc player, a digital audio player, an optical disc file, and the like. In recent years, a next generation DVD or CD having a storage capacity 2 to 6 times that of a current DVD having a storage capacity of 4.7 GB, or a current CD having a storage capacity of 700 MB; or an MO having a mass storage capacity have been developed. The optical information recording/read apparatus has an optical pick-up for detecting a signal track (information pit array) on a disc and information thereon.
The optical pick-up comprises a supporting shaft mounted on a base, and a lens holder rotatably supported on the supporting shaft. A bearing part is provided at the center of the lens holder, and an objective lens is installed on an eccentric position of the bearing part.
If the objective lens of the optical pick-up is dislocated in its optical axis (in radial direction of optical axis), it is impossible to condense optical beams on a correct position of the disc surface and thus impossible to read information correctly. Thus, normally, a focal dislocation of the objective lens is compensated by a focusing servo, and a dislocation of the signal track is compensated by a tracking servo.
In order to increase the density and the capacity of the optical disc, a track pitch and a length of the shortest pit are reduced. In such optical disc with high-density and mass storage capacity, the optical pick-up is required to have much higher response performance of a servo control, especially to have dimensional accuracy of the lens holder.
There is a lens holder on which a plurality of objective lens is mounted for sharing different media in one optical pick-up. In this case, the optical pick-up is required to have much higher sliding-contact characteristic between the lens holder and the supporting shaft, and rotational accuracy to switch one objective lens to another. The lens holder on which a plurality of objective lens is mounted is required to have much higher accuracy of an optical axis. Since a rotating angle becomes wide upon lens switching, and to improve a follow-up of a groove of the optical disc with high-density, it is also required to have much higher sliding-contact characteristic between the lens holder and the supporting shaft, and rotational accuracy. The objective lens and a balancer are mounted on the lens holder with the supporting shaft as the center, thereby deflecting and vibrating the lens holder upon a focus control. To stop the vibration rapidly, it is required to have high attenuation characteristic.
Typically, a conventional lens holder for the optical pick-up is produced by injection molding a resin material.
Referring to FIGS. 9 and 10, a method for forming the conventional lens holder will be described. A mold for injection molding includes a fixed template 16 and a moving template 17, and a cavity 18 is formed therebetween. The cavity 18 includes a cavity 18a for forming a lens supporting shaft, and a cavity 18b for forming a bearing part, and a core pin 19 penetrates the cavity 18b. 
The lens holder is formed using the mold such that the core pin 19 is provided on the moving template 17, a tip of the core pin 19 is inserted into a depression 20 provided on the fixed template 16, the core pin 19 is fixed by the fixed template 16 and the moving template 17, and the resin material is injected through a gate. The gate is either pin point gates 21 in odd numbers shown in FIG. 9, or a side gate 22 shown in FIG. 10.
The lens holder obtained by the above-mentioned forming method cannot have a lens receiving surface and a bearing surface which are formed vertically. This is because a gap is formed between the moving template and the fixed template of the mold to prevent galling when the core pin is inserted, and the gap causes minor dislocation between the core pin provided on the moving template and the depression provided on the fixed template. Due to the dislocation, the core pin is inserted slantingly into the depression when the mold is closed. Accordingly, the lens holder thus formed has the lens receiving surface and the bearing surface which are not vertical. In the optical pick-up including such lens holder on which the lens receiving surface and the bearing surface are provided not vertically, optical beams passing through the objective lens cannot be focused on a disc surface, whereby it becomes difficult to write and detect information.
In addition, when the lens holder on which the lens receiving surface and the bearing surface are provided not vertically is used, it becomes difficult to fix the objective lens, resulting in low working efficiency and high number of defects.
Especially when the lens holder includes two or more of objective lenses, there is a problem that parallelism of lens receiving surfaces cannot be maintained with high accuracy. When the pin point gates 21 in odd numbers as shown in FIG. 9 are used to form the lens holder, a weld line is produced at about center between the gates to decrease roundness of the bearing surface. Therefore, a post-processing is needed to increase accuracy of the bearing part, which leads to problems such as decreased productivity, and increased manufacturing costs. On the other hand, when the material is injected from outside of the cavity 18 using the side gate 22 shown in FIG. 10, the material is injected with different pressures depending on areas, i.e., an area distant from the gate 22, and an area near the gate, resulting in an inaccurate lens receiving surface.
Japanese Patent Publication No. 2886741 describes a method for producing a lens holder, in which gates are provided so that a filler is oriented in an axis direction of an almost all bearing part.
According to the Japanese Patent Publication No. 2886741, it is required to use very small sprues and gate ports which provide the pin point gates around the bearing part with a bearing hole size of 1.0 to 2.0 mm for a light-weighted miniature lens holder currently available. When the sizes of sprues and gate ports are diminished, it becomes difficult to form the mold, and the resin material having a high elastic modulus may easily cause a short shot.
When the pin point gates are provided around the bearing part, and the resin material, especially a liquid crystal resin, is injected, the melted resin with which the bearing part is filled flows to a lens supporting part to easily induce a turbulent flow of the resin at an intersection (bridge portion) of the bearing part and the lens supporting part. As a result, mechanical strength may be decreased.